Ethane and ethylene are formed by the reaction of CH4 and CO2 over unsupported manganese oxide catalysts mainly at 1123 K. CH4 conversion (5-15 mol%) and C2 yield (0.1-0.4mol%) depend greatly on their starting materials. C2 yield with β-MnO2 incrcascs gradually with time on stream, whercas that with e-MnO2 decreases. The yicld with β-MnO2 increases with rise in temperature to a larger extent. Manganese oxide catalysts change chemically in the same manner; β-MnO2 and e-MnO2 are transformed into Mn2O3 in air at 1123 K, then to Mn3O4 upon switching from air to N2, and finally to MnO during reaction. The formation of C2 hydrocarbons is discussed in terms of a redox mechanism involving the lattice oxygen atoms of manganese oxides.